That isn't a fallacy either. If they knew about the NBER or not "they" did not change the definition. The definition they thought was correct is an oversimplification and was not correct.
The continuum fallacy (also known as the fallacy of the beard,[9][10] line-drawing fallacy, or decision-point fallacy[11]) is an informal fallacy related to the sorites paradox. Both fallacies cause one to erroneously reject a vague claim simply because it is not as precise as one would like it to be. Vagueness alone does not necessarily imply invalidity. The fallacy is the argument that two states or conditions cannot be considered distinct (or do not exist at all) because between them there exists a continuum of states.